Electro-optic crystals, such as the aforementioned LiNbO3 and LiTaO3 crystals, are known to change their refractive index in response to electric fields externally applied or developed internally. In addition, these crystals are ferroelectric and, therefore, have a preferred axis (“spontaneous polarization”), whose direction can be changed (“poling”). Oxidic crystals of this kind are needed for a multitude of applications in information and communication technology, for light generation, as data storage devices, and for integrated optics. For example, it is possible to holographically store Bragg gratings in the crystals. These are then used as narrow-band wavelength filters or as reflectors for lasers.
Moreover, so-called “periodically poled” material (“periodically poled lithium niobate/tantalate”, PPLN/PPLT), in which the direction of the spontaneous polarization changes regularly, resulting in the formation of periodically arranged ferroelectric domains, is suitable for building frequency doublers for laser light (SHG, “second harmonic generation”) or optical parametric oscillators (OPOs). LiNbO3 and LiTaO3 crystals are also used as substrate material for waveguides. In particular, these crystals allow integrated optical components to be implemented monolithically together with the applications mentioned above. In this context, the electro-optic effect allows fast modulators to be produced by applying fields to the wave-guiding structure. As is known, doping is used to selectively influence the optical properties of the crystals used.
In particular, frequency conversion in periodically poled material is of great interest for new powerful light sources. Here, the light is often focused in the material to improve the conversion efficiency using high light intensities. Furthermore, the light sources that can be built based on frequency conversion are desired to have high output powers. However, to be able to guarantee reliable operation of the components, these components need to be optimized for use with high light intensities.
A disadvantage here is that high light intensities change the refractive index, and thus the optical properties of the crystal (photorefractive effect or “optical damage”). In particular, inhomogeneous illumination enables charge carriers in the material to move, causing them to be redistributed by drift, diffusion, and/or by the bulk photovoltaic effect, to build up space charge electric fields in the crystal, and thus, to change the refractive index via the electro-optic effect. Impurities in the crystal, which serve as donors or acceptors for the necessary charge carriers and which, as is generally known, are provided by doping, are of importance here. In addition to doping with copper, chromium, or manganese, iron doping, in particular, is widely used and has proven very efficient for the photorefractive effect. On the one hand, this photorefractive effect is useful for recording volume phase holograms in the crystals. For example, the more effectively the material responds to light, the higher is the level of control with which changes in the refractive index can be achieved, thus allowing holograms to be stored, for example, as Bragg gratings.
On the other hand, the optical damage causes deterioration of the optical properties of a crystal. This “optical damage” does not involve any mechanical damage, but merely causes optical inhomogeneities, which affect the propagation of light. Therefore, light can no longer be guided in a controlled manner due to the optical damage. The light is defocused, resulting in losses, especially in waveguides, and leading to a deterioration of the light intensity profiles used. Thus, the components concerned become inefficient.
This can be remedied, inter alia, by increasing the dark conductivity. This makes it difficult for electric fields to build up in the crystal, so that no photorefractive effect will occur anymore. In this connection, reference is made to DE 10 300 080 A1, the contents of which are fully incorporated herein by reference.
However, a particularly high doping level of the crystals of greater than 1×1025 m−3, especially with iron or elements having a comparable effect, increases the dark conductivity to such an extent that it limits the space charge fields. In particular crystals that are doped with a large amount of iron are highly absorptive to visible light. At high light intensities, this high absorption leads to heating, and thus, to thermal expansion, and also to thermally induced changes in the refractive index which, in turn, degrade the beam profile. Moreover, the light loss due to absorption is inconvenient for the applications.
In this context, the iron occurs at least in the two charge states 2+ and 3+. While iron 3+, being an electron acceptor, does not cause absorption in the visible spectral region, iron 2+, being an electron donator, causes a wide absorption band for green to blue light. In order to change the valency state of the incorporated doping centers, it is known to subject the oxidic crystals to a thermal treatment at temperatures around 1000° C. (“annealing”). For example, when annealed in an oxygen atmosphere, the crystals undergo an oxidation process during which the iron is converted from the 2+ valency state to the 3+ state. However, this process cannot be carried out to any desired extent, so that the known method of treatment typically leaves part of the iron in the 2+ state. However, especially in the case of particularly highly doped crystals, this leads to a not insignificant residual absorption. Thus, absorption generally cannot be reduced to absorption coefficients of less than 2 mm−1 using conventional methods.
For other oxidic materials, such as potassium niobate, it is known that thermal treatment in an electric field results in a reduction process instead of an oxidation process. Thus, the change of the charge of the incorporated centers results in a negative valency state.